1. Field of the Invention
The present invention relates to a method for enhancing the outline of shapes that occur in an original image which has been obtained by the use of an electro-optical scanner or similar device for the purposes of preparation for printing.
2. Description of the Prior Art
The use of an electro-optical scanner, or similar device, for the purposes of preparing an image that will be printed or otherwise reproduced is quite common. The preparation steps often include effecting the enhancement of the outline of shapes that occur in the original image, in order to improve the sharpness of the image made by printing or reproduction. The principles for enhancing the outlines of shapes will be described hereinafter.
The original image is divided into many parts. Each part is referred to as a "pixel". An image signal S is produced by optically scanning one pixel from the original image. If value of this image signal S is represented by an 8-bit binary code, the image signal can express any one of 256 degrees of shading.
FIG. 2 shows a series of adjacent pixels scanned from an original image. A center pixel D.sub.22, as shown in FIG. 2, is referred to as the "attention point". An image signal corresponding to the attention point is referred to as the "sharp signal". The 24 pixels surrounding the attention point (D.sub.00 -D.sub.44, excepting D.sub.22) are referred to as the "region adjacent to the attention point". A signal that is produced by averaging (or by weighted averaging) the signal levels of the pixels in the region adjacent to the attention point, is referred to as the "unsharp signal".
Enhancement of outlines is effected by using sharp signals S.sub.xy, and unsharp signals U.sub.xy corresponding to each of the attention points in the matrix of an image D.sub.xy. For example, assume that there is a line of pixels in the original image that may be represented as degrees of shading such as that shown in the original image 100 of FIG. 5. Then a series of sharp signal values, such as that shown the waveform 200 of FIG. 5, will result from scanning the original. Additionally, a series of unsharp signal values can be generated for each of the pixels making up the sharp signal, such that they may be represented by the waveform 300 of FIG. 5. In the waveform 300 of FIG. 5 the outline slopes 302 are gentler than the corresponding slopes in the waveform 200 of FIG. 5. Differential signals (S-U) can be generated by subtracting the unsharp signals U from the sharp signals S. An series of enhancement values ( K.multidot.(S-U) ) can be created by multiplying an enhancement coefficient K by the differential signal S-U as illustrated by the waveform 400 of FIG. 5. Finally, the enhancement values are added to the original sharp signal values to produce the enhanced signal representation shown in the waveform 500 of FIG. 5. By comparing the height of the signals shown in the waveform 500 of FIG. 5 with the height of the original sharp signals shown in the waveform 200 of FIG. 5, it can be seen that the absolute height of the enhanced signal is higher in the slope region 502 of FIG. 5 than in the original sharp signal. This means that the outline of the original image has been effectively enhanced.
This existing method of outline enhancement has the failing that it will operate equally on noisy picture elements, especially any graininess in the original, as well as on the actual shapes in the original image. As a result undesirable noisy elements are also enhanced. To prevent this occurrence, the enhancement coefficient K is effectively reduced to 0 when the absolute value of the differential signal (S-U) is less than a predetermined threshold value T, which is generally called the "Graininess sublimation", in the view that enhancement of graininess in the original is sublimated. The effect of modifying the value of coefficient value K, as just described, is shown in FIG. 6A. In the region A where the absolute value of the differential signal (S-U) is less than the predetermined threshold value T, the enhancement values ( K.multidot.(S-U) ) corresponding to that region A have 0 value. Consequently, outline enhancement of image areas that have relatively small changes in shading (like noisy or grainy areas) is suppressed and the problem described above is solved.
Sublimation of graininess may also be achieved by properly setting up the relationship between the differential signal S-U and the enhanced signal ( K.multidot.(S-U) as shown in FIG. 6B and FIG. 6C. In this fashion, sublimation of graininess can also prevent the unwanted enhancement of noise in the image.
The just described pre-existing methods for enhancing the outline still contain problems that will now be described.
The prior methods of graininess sublimation operate equally upon all degrees of shading. Therefore when noise occurs only at a certain degrees of shading and when the image has small levels of change in shading that occur at different points in the degrees of shading than where the noise is taking place, these small levels of actual image change are suppressed along with the noise. That is, complete enhancement of the desired shapes in the image cannot be effected when they are represented by small levels of shading difference and noise suppression is actively taking place.